Deploying a Unity app to Android phone(s) - Issue.

Hi all.

I'm working on an Android App in the Unity engine and I'm very close to deploying it.

I'm now at the point of testing the games framework on other phones, however, whenever I try install the App from the 'Build' folder onto a phone other than my own I'll get these errors during the installation;

"The app has been interrupted unexpectedly. Please try it again"

or

"Can't open file."

I've insured the API level of the phones I'm testing on is high enough.

This is the first time I've got to the point of deploying an app so its all a very grey area for me. I'm assuming what I'm missing is some kind of installer or something?

Any word of advice or useful links would be much appreciated.

Thank you guys.